1. Field of the Invention
Example embodiments of the present invention relate generally to handling coverage within a wireless communications system, and, more particularly, to methods of call handling based on pilot signals within a wireless communications system.
2. Description of the Related Art
In rural telecommunication systems, base stations are typically deployed so as to cover expansive, outdoor coverage areas. Building structures in rural areas are typically small enough such that a coverage area provided by the outdoor base station is sufficient to support and maintain a communication between indoor mobile users and the outdoor base stations.
In urban areas, however, buildings are built on a much larger scale than rural areas. Large buildings, such as office buildings, warehouses and skyscrapers, may reduce a coverage area provided from outdoor base stations within an interior of a building. Accordingly, indoor base stations are conventionally deployed within such buildings so that the coverage area of a telecommunications system may extend to interior sections of larger buildings.
In conventional code division multiple access (CDMA) systems, indoor and outdoor base stations typically serve mobile users over the same frequency, but with different Pilot Pseudo Noise (PN) Offsets. However, a phenomenon referred to as “pilot pollution” may arise due to the same-frequency implementation of indoor/outdoor base stations. Pilot pollution typically refers to a situation where the mobile received power is relatively high (e.g., a higher value of Ec), but the Ec/Io of the best pilot signal is relatively low. Pilot pollution makes the dominant pilot very unstable and thereby decreases the performance of the mobile. Conventional mobiles operating in accordance with CDMA may search for up to six (6) pilots to maintain in an Active Set (e.g., a set of base stations with pilot signal strengths sufficient to establish a connection) and up to 40 pilots to maintain in a Neighbor Set (e.g., a set of base stations adjacent to base stations within the active set).
A mobile user positioned within a polluted pilot area typically measures the signal strength of all available pilot signals and selects a base station associated with a strongest measured pilot signal. The mobile user then requests to set up a connection with (e.g., if not currently connected to a base station), or handoff to (e.g., if currently connected to a base station other than the selected base station), the selected base station. However, if the signal strength (Ec/Io) of the selected base station is relatively low due to pilot pollution, the call may not be stable. Within a building, certain “exposed” indoor areas may experience increased risk of pilot pollution. For example, higher-level floors in a tall office building may receive the outdoor pilot signal near windows or other exposed areas because the outdoor pilot signal may only be optimized for “street level” connections. Thus, outdoor pilot signals received at mobiles positioned on higher floors of high-rise buildings (e.g., near a window) may be unpredictable and inconsistent. If the mobile utilizes the outdoor carrier or base station to establish call setup or handoff, the call may not be stable and has a higher probability of dropping out. Another example of an exposed area in a building is an entrance and/or exit (e.g., typically positioned on a lower-level floor or lobby). Thus, mobile users may measure the received indoor and outdoor pilot signals in such exposed areas and request either call setup or handoff to the outdoor base station or the indoor base station.
However, the majority of the interior of the building is typically better served by the indoor base stations, while the majority of the exterior of the building is typically better served by the outdoor base station. Accordingly, in an example, if the mobile user connects to the outdoor base station in an exposed area when entering a building, there is a higher probability that the mobile user's call will be dropped if the mobile user moves further into the building and a handoff to the indoor base station is not quickly made. Similarly, in another example, if a mobile user connects to the indoor base station in an exposed area when exiting a building, there is a higher probability that the mobile user's call will be dropped if the mobile user moves further away from the building and a handoff to the outdoor base station is not quickly made.